1. Field of the Invention
This invention relates to a permeameter, i.e. a device for the purpose of measuring the air permeability or fluid permeabilty of porous materials which can notably be in sheet form.
It applies particularly though not exclusively, to the measurement of the air permeability of cigarette papers, wrappers and/or oversleeves.
2. Description of the Prior Art
Generally, it is known that, in order to measure the permeability of a test piece which can consist of an object of any shape whatsoever, two opposite sides of this test piece are subjected to a difference in pressure so as to bring about, within the material included between these two sides, an air stream of which the flow rate is measured.
This difference in pressure is obtained either by generating a partial vacuum at one of the sides while bringing the other side to atmospheric pressure, or conversely, by subjecting one of the sides to air pressure while the other side is brought to atmospheric pressure.
The measurement of the flow of air passing through the test piece is then obtained by maintaining the partial vacuum or pressure applied to the test piece at a constant level, and by measuring by means of a flow meter, the flow of air circulating in the circuit and generating the partial vacuum or pressure as the case may be.
In compliance with ISO 2695 and CORESTA recommendation No. 40, the value VUC in CORESTA unit, (UC being equal to the flow of air (cm.sup.3.min-1) passing through a 1 cm.sup.2 surface of the test piece at a measuring pressure of 1 kPa), of the permeability at a given pressure can then be obtained from the following formula: ##EQU1##
in which:
d is the variance in pressure between the two sides of the test piece (usually 1 kPa) PA1 Q is the flow of air passing through a cross-section A of the test piece (usually 2 cm2) defined by the measuring head.
It so happens that the permeability of objects is very variable, as are therefore the air flows to be measured as a function of the test space area, the ambient test conditions and the applied pressure variance.
Moreover, it is frequently useful to measure, for the same test piece, the flow rate at two different pressures (or partial vacuums), usually 0.25 kPa and 1 kPa, with a view to determining a coefficient (called coefficient of .rect-ver-solid.persillage.rect-ver-solid.) expressing the linearity of the flow/pressure characteristic.
Finally, it may be desirable to plot a graph of the flow/pressure characteristics of the air flow passing through the test piece, especially with a view to determining the exact equation thereof.
Taking into account the fact that, in order to plot such a graph, the pressure (or partial vacuum) must necessarily be varied within a very wide range, the problem stemming from the narrowness of the measurement range of the flow meter is particularly acute. Hitherto, this problem made it necessary for the operator to step in to make scale changes. Total automation of this type of measurement was therefore not conceivable.